1. Field of the Invention
The present invention relates to expandable resin particles consisting mainly of a vinylidene chloride copolymer composition, which is superior in heat resistance, flame resistance, and high-temperature mechanical properties and has gas impermeability and chemical resistance, and to vinylidene chloride-based resin foams superior in heat resistance expanded and shaped in molds from pre-expanded particles which are prepared from said expandable particles. More particularly, the invention relates to foams improved in resistance to thermal deformation, which are expanded and shaped in molds from a non-crystalline vinylidene chloride-based resin composition, having a high glass transition temperature, that is, to molded cellular structures of a novel vinylidene chloride-based resin which are fitted particularly suitable for use as heat insulators and cushioning materials. In addition, the invention relates to expandable particles and pre-expanded particles for the production of said molded foams and to a process for producing those expandable and pre-expanded particles.
2. Description of the Prior Art
Vinylidene chloride copolymers are generally classified into a crystalline group having a crystal structure and a non-crystalline group not having a crystal structure.
The crystalline copolymers contain more than 85 mole % of vinylidene chloride units. Taking full advantage of their superior characteristics including flame resistance, gas impermeability, and chemical resistance, these crystalline copolymers are processed to form products by methods such as melt extrusion, coating in the form of solutions in solvents, coating in the form of aqueous dispersions, and molding in the form of blends with other resins, and are used as fibers, food packaging films, tubes, and cups, etc. However, the crystalline nature of these copolymers, while providing such superior characteristics, is chiefly responsible for problems such as; difficulties in controlling dimensional stability when they are melt-extruded and formed into films; the difficulty of melting them in melt extrusion thereof; the difficulty of dissolving them in solvents to prepare coating solutions and the short life of these solutions; and the short life of aqueous dispersions thereof for coating purposes.
A number of attempts or proposals were made to solve such problems. For instance, an attempt was made to improve extrusion processability by using a plasticizer, but this has been found to bring about adverse effects such that the plasticizer, specially in films may migrate to the film surfaces and contaminate wrapped foods, and that the plasticizer may markedly deteriorate the gas impermeability that is an important characteristic of crystalline vinylidene chloride copolymers.
On the other hand, some attempts were made to reduce the crystallinity that is the main factor causing the above-mentioned problems or to make them amorphous, but it could not be expected using that approach to solve those problems without sacrificing gas impermeability, flame resistance, chemical resistance, and/or heat resistance.
Meanwhile, there have been used synthetic resin foams which have fine uniform closed cells and which are superior in heat insulating properties and cushioning properties, for various purposes according to characteristics that the individual base resins have. In recent years, the practical applications of such foams have drawn attention and this has prompted active studies which are for the purpose of improving the properties synthetic resins, whereby foams of various synthetic resins have become available and techniques of foam processing have been advanced to a great extent. Under such circumstances, there has been a need for foams of a resin constituted mainly of vinylidene chloride which take full advantage of the excellent properties, e.g. the high gas impermeability and flame resistance, of such a resin. Nevertheless, such foams have not yet been developed up to now.
The generally accepted reason for this is that conventional resins constituted mainly of vinylidene chloride have the following drawbacks:
(1) Since the melt-processing temperature of the resin is in close proximity to the decomposition temperature thereof, the resin is liable to decompose in the step of extruding it.
(2) The low permeability of the resin makes it difficult to impregnate the resin with blowing agents.
(3) Since the viscoelasticity of the resin depends greatly upon temperature in the vicinity of the foaming temperature, it is difficult to optimize foaming conditions.
Thus, there have been great difficulties in the production of good, homogeneous, highly-expanded foams from such a resin without decomposing it.
It is already well known how to produce a non-crystalline vinylidene chloride copolymer. This can be accomplished generally by copolymerization under such conditions that the content of vinylidene chloride units in the resulting copolymer is less than 85 mole %.
As an example, Japanese Patent Application Laid-Open No. 44510/85 proposes a non-crystalline vinylidene chloride copolymer for the purpose of solving problems caused by the crystalline nature of vinylidene chloride copolymers. This patent application discloses examples in which copolymers resulting from the copolymerization of vinylidene chloride with methyl acrylate in molar ratios of 47:53 to 81:19 could be melt-extruded and formed into films without adding any plasticizer. However, these copolymers, having low glass transition temperatures, resulted in films or foams having low stiffness at elevated temperatures, hence exhibiting poor resistance to thermal deformation and being insufficient in chemical resistance and flame resistance.
On the other hand, Japanese Patent Publication No. 9551/66 discloses a technique different essentially in technical philosophy from the present invention. The disclosed technique involves processes for producing a copolymer of vinyl chloride with an N-substituted maleimide and for producing a copolymer of these monomers and additionally an ethylenic unsaturated monomer, for the purpose of raising the heat softening temperature of vinyl chloride polymers. That is, it is an object of the technique disclosed in said patent publication to provide vinyl chloride copolymers improved in heat resistance which comprise vinyl chloride and an N-substituted maleimide as essential constituents and, if necessary, an ethylenic unsaturated monomer including vinylidene chloride. Suitable compositions of the copolymer for achieving the above object are disclosed to be 60-90% by weight of vinyl chloride, 1-40% weight of an N-substituted maleimide, and 0-20% by weight of an ethylenic unsaturated monomer. Further, said patent publication teaches that the softening temperature of the homopolymer of said ethylenic unsaturated monomer should be equal or higher than the softening temperature of polyvinyl chloride. That is to say, copolymers of vinylidene chloride are naturally eliminated from the scope of the invention disclosed in this patent since the glass transition temperature of the vinylidene chloride homopolymer is as low as -18.degree. C. Thus, the invention of said patent publication does not suggest the technical philosophy of the present invention, that is, the heat resistance of a vinylidene chloride-based polymer is enhanced while maintaining the high gas impermeability and chemical resistance of this polymer.
Also, Japanese Patent Publication No. 22211/71 discloses a process for producing copolymers constituted of 40-90 mole % of acrylonitrile, 1-10 mole % of an N-substituted maleimide, and 10-50 mole % of at least one ethylenic unsaturated monomer except olefinic hydrocarbons.
The philosophy of the technique disclosed in said published patent application relates to a process for producing acrylonitrile copolymers having improved heat resistance and moldability and does not include a process for producing any copolymer improved in flame resistance and gas impermeability. Even though vinylidene chloride is given as an example of the ethylenic unsaturated monomer, the amount of vinylidene chloride used is limited to a low level. Thus, said published patent application does not suggest of expandable particles of vinylidene chloride copolymers, vinylidene chloride copolymer compositions combining heat resistance, flame resistance, gas impermeability, expandability, and expansion-moldability, or processes for producing such expandable particles or for producing such compositions.
As regards the prior art foam processing of vinylidene chloride-based resins, there are proposed foams produced by extrusion-expansion using specially selected chemical blowing agents (Japanese patent publication Nos. 3968/64 and 16419/67, U.S. Pat. No. 2,948,048, etc.). However, the expansion ratio of these foams are as low as 2 to 3-fold, this cross section is small in size as represented by that of artificial bamboo blind, artificial bamboo blind cores, and ornamental yarns, etc. The purposes of expansion are only for controlling the surface gloss and for imparting flexibility.
A high-expansion technique using physical blowing agents is also known which comprises mixing a finely divided vinylidene chloride-based resin with a physical blowing agent, and extruding and expanding the mixture at relatively low temperatures (about 120-150.degree. C.), thereby yielding foams having densities of about 240 kg/m.sup.3 and higher and cell sizes of about 0.1-1 mm (U.S. Pat. No. 3,983,080). However, these foams are produced from a base resin which shows an X-ray diffraction pattern characteristic of a crystal structure, as disclosed in said patent specification, and hence the resin is difficult to impregnate with the blowing agent and can be impregnated only with a limited amount of blowing agent. Moreover, since the viscoelasticity of the resin melt is not particularly suited for expansion, only foams of relatively high densities, as shown above, can be produced. Furthermore, in this process, the blowing agent with which the resin has been impregnated does not exhibit sufficient expanding effect, thermal decomposition of the blowing agent is difficult to control evenly, and thermal decomposition of the resin results. Hence, extrusion and expansion cannot be continued for enough time and the extruded foam is in string form and has an uneven surface and markedly nonuniform cell sizes.
On the other hand, there are proposed spherical expandable thermoplastic copolymer particles of a single-cell type which have a diameter of about 1-50 .mu.m and contain a volatile liquid blowing agent (Japanese Patent Publication No. 26524/67, Japanese Patent Application Laid-Open No. 59168/74, GBA2,025,429, GB1,588,148, GB1,412,857, GB1,422,827, and EP112,807). The thermoplastic copolymers defined in these patent specifications include a vinylidene chloride-acrylonitrile copolymer and a vinylidene chloride-butyl acrylate copolymer, and some examples in the specifications describe that said particles, when heated to expand and fuse together, form a foam-like structure. However, the inventions disclosed in these patents are quite different from the present invention in the principle of expansion, the structure of expandable particles, the structure and performance characteristics of foam, and hence applications for the foam. Therefore, technical differences between each of these patented inventions and the present invention are rated below to clarify the distinction between them.
The greatest technical difference is as follows: According to the present invention, which is described later in detail, multicellular resin particles having high recovery and resilience are obtained by impregnating resin particles with a blowing agent (dissolved therein), and the formation of an aggregate of these multicellular particles is accomplished by the technique of shaping in a mold, whereby it is possible to obtain a foam having a high proportion of closed cells and superior mechanical strength. In contrast, the expandable resin particles of the above patented inventions are fine balloon-like structures, i.e. so-called micro-balloons, containing a liquid blowing agent and hence the expansion and fusion of these expandable particles by heating results in an aggregate consisting of single-cell balloon-like particle units which have a low proportion of closed cells and also inferior mechanical strength. Moreover, these particles in micro-balloon form are used in mixtures with inks or paints chiefly to form three-dimensional patterns on wall papers and the like, and since these particles have a very small size of 1-50 .mu.m, as stated above, the shaping of them in a mold, when tried, raises problems such that they cannot be uniformly filled in the mold and steam cannot penetrate sufficiently into the molding. Thus, these particles are different fundamentally from the expandable particles of the present invention in that an aggregate cannot be produced from the former particles by expansion and shaping in a mold, this operation being an object of the present invention.
In contrast to the above described prior art, the present inventors previously proposed multicellular foams produced by the technique of expanding and shaping a non-crystalline vinylidene chloride copolymer in molds, said copolymer being constituted of at least 10% of vinylidene chloride and at least one of other monomers copolymerizable therewith, such as styrene, vinyl acetate, vinyl chloride, acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, ethylene, methylpropylene, and methylbutene. In particular, the inventors have found that copolymers of vinylidene chloride with acrylic monomers, above all, are preferable for producing a highly expanded foam. This technique is described in detail in the specification of U.S. Pat. No. 4,613,626.
Based on this technique, high-performance insulating panels have been developed which enjoy the full benefit of high gas impermeability and flame resistance characteristic of vinylidene chloride-based resins. However, since these vinylidene chloride copolymers have low glass transition temperatures, the gas contained in the cells of foams produced from these copolymers expands or contracts with variations in the ambient temperature, causing variations in the dimensions of the foams, and these variations may cause permanent deformation. Particularly at high atmospheric temperatures, there is the significant problem of the secondary expansion due to these temperatures, which tends to cause marked deformation of these foams. In consequence, the applications of these foams are restricted in spite of the superior heat insulating property thereof.
In addition, when the methyl methacrylate content in a vinylidene chloride-methyl methacrylate copolymer, as an example, is increased for the purpose of enhancing the resistance to thermal deformation, this will deteriorate the flame resistance and gas impermeability of the resulting foam characteristic of vinylidene chloride-based resins. Further, it will become impossible to hold Freon gas, which has low thermal conductivity, in the cells over a long term, thus deteriorating the heat insulating property.
That is to say, it is not yet achieved to develop vinylidene chloride-based resin foams of high performance that combine the characteristics of low thermal conductivity, high heat resistance, and high flame resistance.